1. Field of the Invention
This invention relates to radio frequency automatic direction finding (ADF) systems and, particularly, to a quasi-doppler automatic direction finding system utilizing amplitude modulation impressed on the received radio frequency signal. It concerns circular antenna array systems that continuously observe over a complete 360 degrees in azimuth to instantly determine the direction of an incoming signal.
2. Description of the Prior Art
Radio direction finding has long been used by aircraft and marine service as an aid for navigation to determine the location of the receiving station. The ADF systems which are commercially available usually consist of a sense antenna, a loop antenna, and a combiner phase shifter to form a special pattern. This type of ADF operates over a narrow band of one octave or less and is usually limited to a number of discrete frequencies with a special dedicated receiver.
Other methods use mechanical rotation of loops or other directional antennas. These systems require large amounts of power and have all of the mechanical problems associated with physically rotating an antenna. Still other systems derive the bearing information by use of the null, rather than lobe because the null is narrower and will provide more accuracy when adequate signal strength is available. The primary disadvantage of the null type DF or the rotating antenna is that the signal is always fading in and out destroying the intelligence. Another problem with conventional automatic direction finding systems which utilize the null of the antenna pattern is that under weak signal conditions the signal strength approaches the sensitivity threshold of the receiver, thereby broadening the width of the null and limiting the system accuracy. Furthermore the time required to obtain a bearing is on the order of several seconds creating a particular problem with uncooperative transmitters because the signal may not stay on that long.
Another type of automatic direction finding system utilizes the doppler principle. These switching systems require an omnidirectional antenna pattern for each antenna in the array. This usually limits the practical bandwidth to less than one octave because the gain falls off very rapidly both above and below the resonant frequency of the antenna. Of further concern is the number of antennas required for a doppler array. Phasing considerations require antennas no more than a quarter wavelength apart and site error minimization dictates an array as large as possible. As the diameter of the array is increased, the greater number of antennas and connections becomes more and more complicated as well as requiring higher scanning rates and more antenna drivers.